Lactobacillus acidophilus is a Gram-positive, rod-shaped, non-spore forming, homofermentative bacterium that is a normal inhabitant of the gastrointestinal and genitourinary tracts. Since its original isolation by Moro (1900) from infant feces, the “acid loving” organism has been found in the intestinal tract of humans, breast-fed infants, and persons consuming high milk, lactose, or dextrin diets. Historically, L. acidophilus is the Lactobacillus species most often implicated as an intestinal probiotic capable of eliciting beneficial effects on the microflora of the gastrointestinal tract (Klaenhammer and Russell (2000) “Species of the Lactobacillus acidophilus complex” Encyclopedia of Food Microbiology, Volume 2, pp. 1151-1157. Robinson et al., eds. (Academic Press, San Diego, Calif.). L. acidophilus can ferment hexoses, including lactose and more complex oligosaccharides, to produce lactic acid and lower the pH of the environment where the organism is cultured. Acidified environments (e.g., food, vagina, and regions within the gastrointestinal tract) can interfere with the growth of undesirable bacteria, pathogens, and yeasts. The organism is well known for its acid tolerance, survival in cultured dairy products, and viability during passage through the stomach and gastrointestinal tract. Lactobacilli and other commensal bacteria, some of which are considered as probiotic bacteria that “favor life,” are generally recognized for their role in flavor and aroma development and to spoilage retardation in fermented food products, and have been studied extensively for their effects on human health, particularly in the prevention or treatment of enteric infections, diarrheal disease, prevention of cancer, and stimulation of the immune system.
During fermentation, lactic acid bacteria are exposed to toxic byproducts of their growth, such as lactic acid and hydrogen peroxide, antimicrobial agents produced by neighboring microorganisms, and the harsh environmental conditions that is encountered during proper fermentation of a raw food item. They must also adapt to the extreme conditions found in the stomach during ingestion, and severe temperatures associated with storage or production conditions, as well as compete with other microorganisms for resources. These bacteria have evolved sensory and regulatory mechanisms, which enable them to monitor external conditions and respond accordingly. One such mechanism is referred to as the “two-component” system, and is structured around two proteins: a histidine protein kinase and a response regulator protein. Furthermore, one of the major responses controlled by these sensory and regulatory systems of these bacteria is the production of their own antimicrobial agents, of which bacteriocins are an example. Two-component regulatory systems have been shown to control many diverse processes in bacteria, such as sporulation, chemotaxis, nitrogen assimilation, outer membrane protein expression, response to osmolarity, regulation of competence and virulence, as well as the production of antimicrobials.
Microorganisms that can respond to changes in the environment, such as those present during commercial fermentation and storage, as well as those microorganisms that can compete more effectively with other microorganisms are advantageous. Therefore, isolated nucleic acid sequences encoding these proteins are desirable for use in engineering microorganisms, including Lactobacillus acidophilus, to have an increased ability to tolerate changes in growth environment and an improved ability to inhibit food-borne pathogens.